How do I use the Rule of Three Calculator?
Enter your values in the fields and the result updates instantly as you type.
Rule of Three Calculator
Solve proportions quickly: if a relates to b, find x for c. Enter your values, review the formula, and use the worked result as a quick check for everyday planning.
Rule of Three Calculator
Solve proportions quickly: if a relates to b, find x for c.
Any numeric value (not zero)
Any numeric value
Any numeric value
Results
x
30
Formula: x = (b * c) / a
Rule of Three: Solve Proportions Quickly
The rule of three (cross-multiplication) solves problems where you know three values and need to find the fourth in a proportion. Used for scaling recipes, converting units, calculating doses, and solving ratio problems.
Formula
If a relates to b as c relates to x, then x = (b × c) ÷ a. The calculator expects a (linked to b) and c (linked to the unknown x). a must not be zero.
Worked example
If 2 apples cost 10, how much do 6 apples cost? a=2, b=10, c=6. x = (10 × 6) ÷ 2 = 30.
Practical guidance
Setting up the proportion correctly
Identify which pairs correspond. If 5 workers take 8 days, how many days for 2 workers? 5 workers → 8 days, 2 workers → x days. a=5, b=8, c=2. x = (8×2)÷5 = 3.2 days. Check that the relationship is proportional (inverse relationships need different setup).
Direct vs inverse proportion
The rule of three shown here assumes direct proportion (both values increase together). For inverse proportion (one increases as the other decreases), set up differently: if a×b = c×x, then x = (a×b)÷c.
Frequently asked questions
What formula does this calculator use?
x = (b * c) / a
Can I use decimal values?
Yes. Decimal inputs are supported for most calculators. Integer-based tools use rounded integer values where appropriate.
Is this calculator free?
Yes. CalcuNimble calculators are free and optimized for fast everyday use.